Interpretive Summary: After hearing about a “supposed natural European hybrid walnut,” Luther Burbank began making controlled crosses between walnut species in the late nineteenth century. He first crossed Juglans hindsii (northern California black walnut)× J. regia (Persian walnut) that produced progeny that he named ‘Paradox’ because of the extremely fast growth and other “anomalies.” He also crossed two American species, J. hindsii (Northern California Black Walnut)× J. nigra (eastern black walnut), producing ‘Royal’ walnut progeny that were fast growing, prolific nut producers. A third interspecific hybrid was a cross between J. ailantifolia (Japanese walnut and J. regia. This also resulted in extremely fast growing progeny, but they were not named like the other two hybrids. He observed segregation in the F2 populations and described giants and dwarfs. Luther Burbank also made selections for walnut scion cultivars and was especially interested in thin shelled nuts. He collected seeds from a J. regia growing in San Francisco because it produced regularly and had very high quality nuts with relatively thin, but poorly sealed shells. He selected one of its seedlings as the new cultivar Santa Rosa Soft-Shell. He described the cultivar as bearing large crops of nuts that are nearly white with thin shells and delicious white meat. Burbank’s contributions to the walnut industry endure to this day, especially through the widespread use of ‘Paradox’ walnut rootstock.

Technical Abstract:
After hearing about a “supposed natural European hybrid walnut,” Luther Burbank began making controlled crosses between walnut species in the late nineteenth century. He first crossed Juglans hindsii (northern California black walnut)× J. regia (Persian walnut) that produced progeny that he named ‘Paradox’ because of the extremely fast growth and other “anomalies.” He also crossed two American species, J. hindsii (Northern California Black Walnut)× J. nigra (eastern black walnut), producing ‘Royal’ walnut progeny that were fast growing, prolific nut producers. A third interspecific hybrid was a cross between J. ailantifolia (Japanese walnut and J. regia. This also resulted in extremely fast growing progeny, but they were not named like the other two hybrids. He observed segregation in the F2 populations and described giants and dwarfs. Luther Burbank also made selections for walnut scion cultivars and was especially interested in thin shelled nuts. He collected seeds from a J. regia growing in San Francisco because it produced regularly and had very high quality nuts with relatively thin, but poorly sealed shells. He selected one of its seedlings as the new cultivar Santa Rosa Soft-Shell. He described the cultivar as bearing large crops of nuts that are nearly white with thin shells and delicious white meat. Burbank’s contributions to the walnut industry endure to this day, especially through the widespread use of ‘Paradox’ walnut rootstock.